This invention relates to an apparatus and method for the inspection of objects and, in particular, to an apparatus and method for automatic three-dimensional (3-D) inspection of such objects.
In many applications today, the inspection of objects or assembled parts is carried out manually. However, manual inspection is often inconsistent due to a variety of factors which tend to impair the ability of the human inspector. These factors include tiredness, ill health or distractions caused by mental state, surroundings or otherwise. Also, manual inspection is costly and tedious.
Because of these disadvantages, attention has been directed toward automating the inspection process. Unfortunately, inspection of objects requires the exercise of a great deal of judgment and the ability to view an object from many angles. These requirements make the process complex and have impeded the development of automatic inspection systems, especially for applications where manual inspection is satisfactory.
The prior art inspection systems developed to date have extensively used two-dimensional (2-D) machine vision or sensors and have proved successful in certain applications. These sensor systems have also incorporated mechanisms for measuring the reflectance of the object to gather additional data for making decisions. This is accomplished by measuring the brightness of the reflected light to form a gray-scale which can be used to define part boundaries. Pixels, unit areas within the sensor field of view, are counted for each gray scale level found and the results fit to a template of acceptable parts or objects to provide a basis for a statistical decision.
X-rays have also been used for inspecting objects and, in particular, for inspecting solder joints or connections. These x-ray systems use techniques similar to the 2-D techniques discussed previously. Also, solder joint inspection has been carried out using laser pulse heating in conjunction with a thermal imaging infrared sensor. In this case, the heating and cooling profile are compared with that of a known solder joint in order to decide if the joint is acceptable.
Solder joint inspection is an example of a broad class of objects and parts for which the inspection problem is compounded due to the irregular contoured features of these parts. Such irregularities make it difficult to define the inspection procedure and acceptance criteria. In particular, since a solder joint is not a machined part, no template is precisely specified that can be used for its inspection.
Accordingly, single feature determination is not adequate for inspection of solder joints and other like irregular parts. Many features must be considered and the features must be considered in associated combinations. Basic to these considerations is the ability to accurately measure the features so that pass/fail determinations can be made. By measuring the 3-D rather than 2-D characteristics of the features, a great deal of the ambiguity can be removed from the inspection process. U.S. Pat. No. 4,553,844 is an example of a system which uses 3-D analysis and triangulation techniques for solder joint inspection. The '844 patent points out that in systems of this type, where optical triangulation is used to make 3-D measurements, mirror-like surfaces produce specular reflections that increase the difficulty of measurement by producing false images.
Another discussion of the application of structured light to produce 3-D measurements for inspection can be found in SPIE, Volume 336, Robot Vision (1982) page 121 through 127 entitled, "Automatic Visual Inspection of Solder Joints on Printed Circuit Boards". Various common defects are described which the inspection system must detect since such defects normally are not revealed by electrical test.
It is an object of the present invention to provide an improved apparatus and method for the inspection of assembled parts or objects and, in particular, the inspection of solder joints.